It is a matter of common knowledge that plastic containers are widely used for packaging some foodstuffs, medicinal and pharmaceutical products as well as a host of other items. It is also a matter of common knowledge that after the container is filled or packed with some comestibles, the container is sealed and thermally processed in order to sterilize its contents. Thermal processing of the plastic containers is carried out at elevated temperatures of as high as about 275.degree. F. to ensure proper sterilization so that the comestibles will be safe for human consumption.
Several factors must be taken into consideration when forming a plastic container for packaging foods. These factors include resistance to oxygen permeation and, product migration, and structural integrity. Accordingly, plastic containers are frequently formed from multilayer laminates which include an intermediate oxygen barrier layer and inner and outer polyolefin structural layers. Adhesive layers may be used to bond the inner and outer layers to the barrier layer, and other layers may be optionally included, if desired. Where resistance to oxygen permeation is not essential, the plastic container may be made of a single layer of a suitable polymer such as polyethylene, a blend of polyethylene with polypropylene or a copolymer of ethylene with propylene.
A typical multilayer laminate used in making plastic containers comprises an intermediate oxygen barrier layer of ethylene-vinyl alcohol copolymers; an outer polyolefin layer, preferably polyethylene, polypropylene or a blend thereof, or a copolymer of ethylene with propylene; and an inner polyolefin layer similar to the outer layer. For this reason, this layer will hereinafter be sometimes referred to as the "contact layer", or when forming the inner wall of a container, as the "contact wall".
When the contact wall or contact layer of the plastic container is made of polyethylene, or a polymer or other material containing polyethylene, or ethylene copolymerized with another olefin (e.g., propylene); or if the container contact wall comprises any of the foregoing materials, after sterilization of the container, an off-flavor is often detected in the thermally processed comestibles. This off-flavor, which is variously characterized as "burnt smokey", "oxidized", or by some other appropriate designation by experienced members of a food taste panel, is often objectionable to consumers.
Accordingly, it is an object of this invention to substantially reduce the off-flavor taste of comestibles which have been thermally sterilized in plastic containers.
It is also an object of this invention to use plastic containers to package foods and other items wherein the container contact wall or contact layer does not impart off-flavor tastes to the container content during hot-filling or thermal sterilization, or wherein such off-flavor tastes are substantially reduced.
It is yet another object of this invention to form such plastic containers with contact wall made of a polymer which does not adversely affect the flavor of the packaged foods when the container is subjected to thermal processing.
The foregoing and other objects, features and advantages of this invention will become more evident from the ensuing detailed description of the invention.